In many electronic products such as laptops, desktops and PDAs (Personal Digital Assistant), DC sources are needed to supply power to each functional module. As COT (constant on time) DC-DC converters have advantages of fast transient response, simple structures and so on, they are widely applied in electronic products.
Ramp compensation is generally required for COT DC-DC converters to stabilize an output voltage. In prior art, a ramp compensation signal has a constant slope. When a power switch in a COT DC-DC converter is turned on, the ramp compensation signal is reset to zero, and then increases with the constant slope. When the output voltage decreases to the sum of the ramp compensation signal and a reference voltage, the power switch is turned on, and the ramp compensation signal is reset to zero again. This process repeats in every switching cycle.
However, when a load steps up, as the output voltage decreases rapidly, a feedback control loop of the COT DC-DC converter generates a cluster of pulses in a short time. Due to the cluster of pulses, an inductor current increases sharply, and the output voltage is then pumped higher, thus an overshoot in the output voltage is induced. In some cases, there is even a ring back in the output voltage after the overshoot. Accordingly, it is necessary to suppress the overshoot and the ring back in the output voltage to improve the performance of load transient response.
Also, in some applications, it is required that the output voltage decreases as the output current increases, so as to reduce the peak-to-peak value of the output voltage during the transient process to improve the stability.
That is to say, DC droop is needed in the aforementioned applications and similar situations.